Mercaptoalkylsilanes may be prepared in a one-step process by reacting (haloorganyl)alkoxysilane compounds with thiourea and ammonia (DE AS 2035619). This method has the disadvantage that long batch times (more than 24 hours) are required to achieve economically acceptable conversion rates. The yields that are achievable with this procedure are variable and typically reach values of only 75 to 80%. In addition, guanidine hydrochloride is formed and this must be separated and disposed of at additional expense.
Mercaptoalkylsilanes may also be prepared in which mercaptosilanes are formed by hydrogenation of thiopropionic acid amide silanes under pressure (EP 0018094) or by hydrogenation of cyanoalkylsilane compounds in the presence of elemental sulfur or hydrogen sulfide (U.S. Pat. No. 4,012,403). Both processes have the disadvantage of poor yield.
U.S. Pat. No. 3,849,471 discloses the preparation of mercaptosilanes by reaction of (haloorganyl)alkoxysilane compounds with hydrogen sulfide in the presence of ethylene-diamine and large amounts of heavy metal sulfides. A disadvantage of this process is the formation of various secondary products which must be separated out. The process can be improved by eliminating the diamines and reacting the starting silanes with hydrogen sulfide in the presence of ammonia, primary, secondary or tertiary amines, and, optionally, in the presence of polar, protic or aprotic media (U.S. Pat. No. 4,082,790). However, in order to achieve the required reaction temperatures, the process must be carried out in high-pressure autoclaves. If the reactions are carried out in the absence of polar media, long reaction times are needed to achieve acceptable conversion rates. Moreover, the metered addition and handling of highly toxic H2S on an industrial scale is undesirable, expensive and requires rigorous safety precautions.
Mercaptosilanes may also be formed by the reaction of alkali hydrogen sulfides with (haloalkyl)alkoxysilanes in a methanolic medium (GB 1 102 251). This procedure has the disadvantage that an extraordinarily long reaction time (96 hours) is required to achieve high conversion rates and the yield achieved is unsatisfactory.
(Mercaptoalkyl)alkoxysilanes may be produced by reacting alkali hydrogen sulfide with suitable (haloalkyl)alkoxysilanes in the presence of a 10–100% molar excess of H2S (U.S. Pat. No. 5,840,952). However, on an industrial scale, this process has the disadvantage that highly toxic H2S has to be stored, metered and handled. In addition, the process is carried out in two stages and this results in a diminished space-time yield.
Another process for preparing (mercaptoalkyl)alkoxysilanes is by reacting (haloalkyl)alkoxysilanes with alkali hydrogen sulfide (NaSH) in polar, aprotic solvents (EP 0 471 164). One disadvantage of this process is that it uses a large quantity, at least 50 vol. %, of solvent, and this may be highly toxic, e.g., dimethylformamide. In addition, the high boiling-point of solvents such as dimethylformamide makes the later distillative reprocessing and purification of the reaction products more difficult.